This invention relates to the field of semiconductor devices usable as optical energy to electrical energy transducers or solar cells and to the efficient fabrication of such devices.
The vertical-junction solar cell was proposed in the early 1970's, and in one arrangement thereof is described in U.S. Pat. No. 3,690,953. Such solar cell designs were earlier found to be especially desirable in their ability to minimize the degradation of power generation capability caused by ionizing radiation damage in an aerospace environment. In the evolution of these devices, an article published by one Patrick W. Rahilly, and titled "Vertical Multijunction Solar Cells," and appearing in the May 1972 Ninth IEEE Photovoltaic Conference Record at page 85 describes an improved vertical multijunction solar cell which possesses a pn-junction linear density of 2000 junctions per centimeter.
Although this improved solar cell design also proves to have desirable radiation hardening characteristics, cells of the reported type, are found to possess several major disadvantages-disadvantages which are, for example, described in the additional published article "Vertical Multijunction Solar Cell Fabrication", appearing in the Conference Record of the Tenth IEEE Photovoltaic Conference, pages 194-195, 1973. Such solar cells, for example, typically have a significant recombination current loss which results from their large pn-junction periphery, have poor power generation efficiency at short incident light wavelengths, and have metal contacts which are difficult to fabricate with desirable reliability due to their closely spaced pn-junction regions.
As a result of embodying the 1973 article's work, conversion efficiencies as large as 5.5 percent are observed. Analysis reveals that this relatively low performance can be attributed to the large series resistance resulting from long path lengths that the cell's generated carriers must traverse prior to collection by the surface-mounted, metallized-grid contact structure of the cell array. A further effort supported by the Solarex Corporation of 1335 Piccard Drive, Rockville, Md. 20850 additionally optimizes the vertical multijunction solar cell, achieves conversion efficiencies greater than 15 percent, and is reported in a U.S. Government report, AFWAL-TR-Bl-2052 titled: "Silicon Solar Cell Optimization," dated June 1981, available from the Defense Technical Information Center (DTIC), Defense Logistics Agency, Cameron Station, Alexandria, Va. 22304-6145 as AD A106005 (92 pages). Reliability tests reveal, however, that the walls of the improved vertical pn-junction solar cell are structurally weak and can fracture when thermally cycled between the temperatures of 77.degree. K and 373.degree. K, for example.
Several arrangements have also been proposed to improve the mechanical robustness of the vertical multijunction solar cell; these arrangements include, for example, the cells of U.S. Pat. No. 4,409,423, and U.S. Pat. No. 4,420,650, which are described below. While each of these designs possess superior mechanical strength, both manifest conversion efficiencies which are often less than 2 percent.
One subsequent proposal to increase the conversion efficiency of a solar cell incorporates an interdigitated back-contact feature; such cells are described in the article "The Interdigitated Back-Contact Solar Cell. A Silicon Solar Cell For Use in Concentrated Sunlight", which appears in the IEEE Transactions on Electron Devices, vol. ED-24, pages 337-342, April 1977. While this design eliminates the self-shading losses contributed by a solar cell's surface conductor grid, its long pn-junction carrier diffusion lengths make it undesirably radiation sensitive.
The present invention, however, achieves increased solar cell conversion efficiency without sacrificing radiation hardening and structural strength. The invention, moreover, combines the vertical-channel concept with the interdigitated back-contact concept to achieve a cell of significantly increased practical utility. The disclosed cell arrangement also totally eliminates self-shading of the active surface, has minimal reflection losses, minimizes the undesirable carrier recombination current loss, is amenable to silicon fabrication processes, and enhances the physical robustness of the solar cell array.
The prior patent art includes several examples of inventions relating in a general sense to the present invention. Included in these is U.S Pat. No. 4,420,650 issued to J. F. Wise et al, a patent also assigned to the U.S Air Force. The Wise et al '650 patent is especially concerned with a vertical-junction solar cell that is fabricated on (110)- oriented silicon and in which the light transducing surfaces are disposed along the length of the wafer's transversing grooves-in accordance with a predetermined relationship between the groove's depth and the individual solar cell element's length. The Wise et al patent is also concerned with differing etch rates in differing crystallographic planes and with the use of potassium hydroxide as an orientation-dependent etchant.
The Wise et al patent also shows disposal of individual solar cell elements in parallel grooves with a 50% lengthwise displacement of cells in adjacent grooves. Notwithstanding these points of similarity between the Wise invention and the solar cell described herein, it is to be noted that the Wise et al apparatus is concerned with a solar cell in which the individual cell cavity is relatively shallow with respect to the host wafer's thickness, employs metallic electrical contacts on both the top and bottom surfaces of the solar cell structure, does not realize the advantages of the pointed top surface cavity walls, has lower pn-junction density, and does not realize the advantages of the present invention cell cavity etch termination arrangement in its fabrication.
The present invention therefore, adds a significant contribution to the solar cell art over the disclosure of the Wise et al '650 patent.
Another patent by J. F. Wise, also assigned to the U.S. Air Force, U.S. Pat. No. 3,690,953 describes one of the earlier arrangements of a vertical pn-junction solar cell and involves the side by side placement of p- and n- semiconductor materials together with an overlay of current collecting and conducting metallic paths disposed on the cell's light receiving surface. The surface disposition of the current conducting stripes and the absence of individual solar cell cavity structures are significant points of distinction between the '953 patent and the present invention.
Another patent concerned with solar cells and similar devices, U.S. Pat. No. 4,101,351, in the name of P. L. Shah et al, involves pn-junction formation arrangements, anti-reflective coatings, and improved metallization arrangements for silicon solar cells of both the planar disposed and vertical multijunction cell types. The use of both front- and back-contacts in the '351 patent is a notable distinction from the present invention.
The U.S Pat. No. 4,409,423 of J. F. Holt is also concerned with vertical-junction solar cells and provides a cell of increased mechanical robustness. The use of front face metallic contacts and the disposition of cells into a wafer hole structure are particularly notable points of distinction between the '423 patent and the solar cell of the present invention.
The patent of R. N. Hall, U.S. Pat. No. 4,427,839, is concerned with a solar cell having rear face electrodes and a frontal surface that is disposed in the form of pyramidal apertures which include a material with a high index of refraction filling the pyramidal apertures. Both the pyramidal aperture structure and the disclosed (100) crystallographic orientation of the substrate are points of notable distinction between the Hall solar cell and that of the present invention.
The U.S. Pat. No. 4,478,879 of C. R. Baraona et al, is also concerned with back-contact solar cells and a method for fabricating such cells. In addition to the planar disposition of the '879 solar cell, the use of screen printing in the cell fabrication process provides notable distinction between the present invention and the '879 solar cell.
In addition, the U.S. patent application Ser. No. 07/282,423 filed Dec. 9, 1988 by E. S. Kolesar Jr. and titled "Hybrid Wafer Scale Microcircuit Integration" is concerned with the disposing of cavities in the surface of a semiconductor wafer by orientation dependent etching. The shape utilization and filling of the cavities in this application differ from the present inventions, however.